This application claims the priority of German Patent No. 197 298 92.3, filed Jul. 12, 1997, the disclosure of which is expressly incorporated by reference herein.
The invention relates to an electromagnetically-controlled measuring apparatus for the volumetric measurement of the quantity of fuel injected by a fuel-injection pump that has a displacement transducer which is surrounded by cooling chambers. The apparatus also includes a differential coil pair and a magnetically conductive core, which can be guided axially in the cooling chambers. The coil and the conductive core constitute part of a lifting rod connected to a measuring piston, with a coil carrier and a housing surrounding the coil carrier.
Such a measuring apparatus is known from German patent document DE 39 16 419 C2. This measuring apparatus essentially consists of a measuring piston, which is acted upon by gas pressure. A lifting rod is guided in a measuring cylinder and separates a measurement chamber from the gas pressure chamber. Additionally, the apparatus has a displacement transducer which is surrounded by cooling chambers and soldered to the lifting rod, and a differential pair of coils and a magnetically conductive core (a ferromagnetic iron core) which can be shifted axially therein.
When the quantity of injected fuel is measured with this measuring apparatus, which is intended for use with diesel injection systems, the linearity of the displacement transducer and the independence of the scale factor from the temperature must satisfy high requirements. However, in the case of injection systems, the sensitivity of the displacement transducer unavoidably changes as a function of the temperature. This is because these systems are operated at high and varying pressures, that is, direct injection systems such as systems working according to the common rail principle. With such systems, the temperature of the fuels, which are injected under high pressure, changes very rapidly with the respective pressure.
It is an object of the invention to further increase the accuracy of the measurement by the piston of the generic measuring apparatus. In addition, the measuring apparatus shall be particularly suitable for injection systems, which are operated at high pressures, such as common rail systems.
This and other objects and advantages are achieved by the electromagnetically-controlled measuring apparatus according to the invention, in which eddy currents are suppressed by using a ferritic material for the magnetically conductive core, and by using a lifting rod as a sleeve which cannot be magnetized. This sleeve tightly surrounds the ferritic core and is provided with a continuous slot for interrupting the circular current paths. In addition, in order to avoid eddy currents, the housing which surrounds the coil carrier is also made of a ferritic material or a non-magnetic material. The sleeve is equipped with numerous longitudinal grooves, which open toward the coil carrier and accommodate ferritic rods. As a result, the temperature dependence of the scale factor is reduced to the desired extent which yields a higher measuring accuracy.
Moreover, the measurement is more precise due to both displacement transducer coils being divided into three sections. In each case, the outer section is carefully wound from winding to winding, with a high number of layers.
In an embodiment according to the invention, only the middle section of the transducer coil is wound with a low number of layers.
In still another embodiment according to the invention, to achieve an extremely good linearity, only the length of the core is optimized.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.